1. Field of the Invention
This invention generally relates to a bicycle crank arm. More specifically, the present invention relates to a bicycle crank arm with a sensor that detects a pedaling force.
2. Background Information
Bicycles are sometimes equipped with various sensors for providing information to the rider and/or for providing information to a controller to control various aspects of the bicycle such as shifting or suspension stiffness. One such sensor is a torque or force sensor for detecting a pedaling force of the rider. Various sensing arrangements have been proposed for detecting a pedaling force of the rider. For example, in U.S. Pat. No. 7,516,677 (assigned to Shimano Inc.), a cylindrical torsion-detecting sleeve member (force sensor unit) is provided on a crank axle for detecting a pedaling force applied to the crank axle during pedaling. Another example of a force sensor for detecting a pedaling force applied to a crank arm is disclosed in Japanese Utility Model Patent No. 3047816 (see FIGS. 3 and 4). In this utility model patent, a torque sensor is used to detect a relative resistance between a flexible rod and a crank arm. The crank arm and the flexible rod are rotatable around a crank axle via a one-way clutch bearing. A free end of the flexible rod contacts a portion 12 of the crank arm.
More recently, strain gauges have been used to detect a pedaling force during pedaling. For example, a force sensor is disclosed in U.S. Patent Application Publication No. 2010/0282001 (assigned to Shimano Inc.), which utilizes a strain gauge for detecting a pedaling force applied to a crank axle during pedaling. Another example is disclosed in U.S. Pat. No. 8,006,574 where a strain gauge is used on a crank arm to detect a pedaling force by a rider. Typically, the strain gauge needs to be installed with a high accuracy to obtain accurate measurements. Thus, the manufacturing costs and/or manufacturing time in manufacturing a bicycle crank arm increases when a strain gauge is installed thereon to detect a pedaling force by a rider.